1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device, particularly to a potential generation circuit of a semiconductor storage device using a ferroelectric capacitor.
2. Description of the Related Art
In a ferroelectric capacitor, an applied voltage and a polarization amount have a hysteresis characteristic, and the capacitor has residual dielectric polarization when the applied voltage is zero. A ferroelectric random access memory (FeRAM) using the characteristics has been known. Data is written or read, when a voltage generated between a plate line and a bit line is applied to the ferroelectric capacitor. The data read into the bit line is amplified by a sense amplifier.
For the ferroelectric capacitor, several characteristics change in accordance with temperature, and one of the characteristics correspond to a hysteresis characteristic. FIG. 33 shows the hysteresis characteristic at a low temperature, and FIG. 34 shows the hysteresis characteristic at a high temperature. As shown in FIGS. 33, 34, a voltage (saturated voltage) VTL required for saturation of a residual dielectric polarization amount and the polarization amount increases at the low temperature, therefore hysteresis increases. The residual dielectric polarization amount and saturated voltage VTH decrease at the high temperature, therefore the hysteresis decreases.
On the other hand, a potential applied to the plate line has been a fixed value regardless of the temperature. Therefore, a plate line driving potential at the low temperature needs to be used in order to assure normal write and read of the data regardless of the temperature. As a result, a voltage VA which is greater than the saturated voltage VTH at the high temperature is applied to the ferroelectric capacitor at the high temperature. This excessive voltage does not contribute to the increase of the written polarization amount at all, and is useless. Additionally, imprint of the hysteresis characteristic (shift of hysteresis) or fatigue of a ferroelectric film occurs. This causes a deterioration of reliability of a ferroelectric memory.
Time required for polarization reverse (polarization reverse time) is another one of the characteristics of the ferroelectric capacitor which change at the temperature. FIGS. 35, 36, 37 show the polarization reverse time at 70° C., 25° C., −25° C. Curves in the drawings show differences of the applied voltages. As shown in FIGS. 35 to 37, a time required until the polarization amount is saturated after the polarization reverse is short at the high temperature, and the time is long at the low temperature. Note that FIGS. 35 to 37 relate to an SBT (SrBi2Ta2O9) film, and this also applies to a PZT (Pb(Zr,Ti)O3) film.
On the other hand, a time for driving the plate line and a time for activating the sense amplifier have been a fixed values regardless of the temperature. FIG. 38 shows a timing chart of major nodes of a conventional ferroelectric memory. As shown in FIG. 38, the data is read and written involving the polarization reverse of data “1” from t31 for the driving of the plate line until t32 for the sense amplification, and from t33 for a drop of a plate line driving potential to VSS until t34 for sense amplifier inactivation. The polarization reverse time at the low temperature needs to be used in order to assure a series of normal operation regardless of the temperature. As a result, the voltage continues to be applied over the polarization reverse time at the high temperature or longer at the high temperature. This causes the imprint and fatigue, and deteriorates the reliability of the ferroelectric memory.